The present invention relates to a video display apparatus and more particularly to a video display apparatus provided with a CRT (cathode ray tube).
Systems using a display apparatus as a control input medium have been widely used in various kinds of electronic control devices such as a computer.
Conventionally, characters, figures, etc. were displayed on a display in such a manner that the characters or figures to be desired to display are caused to light up on a black background. With improvement of performance in the display device, however, in some cases, a scheme in which characters and figures to be desired to display are displayed with black, gray, and color on a white-shining back ground as if the characters and figures were drawn on a sheet of white paper has been adopted mainly in a word processor and office computer (small business computer) for business processing. So-called `paper whites` or display devices in which characters and figures are displayed in black or gray on a white back ground due to a white display and in particular, a monochromatic display device using CRT with white fluorescent substance applied (paper white CRT) has been widely used because of its low cost and excellent quality.
Meanwhile in a general purpose computer system, such as a small business computer, personal computer, etc. the display device used, which is connected with a control device through a cable, is adapted to be removable so that a user can select the display device in accordance with his use object from many kinds of display devices such as a color CRT, monochromatic (green) CRT, liquid crystal display, plasma display, etc. In such a system, the manner of display is changed in accordance with the display device connected. For example, the paper white CRT has a display format "background: white, character/figure: reverse display of black (black display on white background)", and the green CRT has a display format "background: black, character/figure: normal display of green (green display on black background)." Further, a video signal and a synchronization signal are transmitted as control signals for display; the video signal is transmitted for only a display area (valid or effective display area) which can be controlled by a control program.
In such a system, "black display on white background" in the white CRT has been implemented by a control program which serves to whiten the background and display characters or figures in black.
On the other hand, the display device with a CRT has drawbacks on its outer periphery that large distortion occurs, the display is out of focus, and the display size changes in accordance with the contents to be displayed and conditions under which the display is used. To obviate these drawbacks, this display device adopts a under-scan display scheme in which the effective display area is made one-size smaller than the aperture size of the display tube of CRT. Further, the monochromatic liquid crystal display is constructed so that the valid display area is kept to be substantially equal to the aperture size of the display, whereas in view of insufficient display quality the display format is made to be changeable between "black display on white background" and "white display on black background" whereby a user can select an easy-to-see display image.